A9F8470Dd01 - 1168 G. Mazzanti: The Combination of...

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G. Mazzanti: The Combination of Electro-thermal Stress, Load Cycling and Thermal Transients and its Effects 1070-9878/09/$25.00 © 2009 IEEE 1168 The Combination of Electro-thermal Stress, Load Cycling and Thermal Transients and its Effects on the Life of High Voltage ac Cables G. Mazzanti Department of Electrical Engineering, University of Bologna, Viale Risorgimento, 2 Bologna, 40136 Italy ABSTRACT The paper proposes a procedure for life estimation of high voltage ac cables in real operating conditions, i.e. subjected to voltage and load cycles, so that electrothermal stress is the dominant aging factor of cable insulation. As possible alternatives for representing the effects of the electrothermal aging of insulation, three life models well- known in literature are considered, namely the Zurkov, Crine and Arrhenius-IPM models, each within the probabilistic framework needed for associating time-to-failure with reliability. The cumulation of loss-of-life fractions over load cycles is evaluated through Miner’s law. The thermal transients that affect insulation as a consequence of cyclic current variations are simulated via the CIGRE two-loop thermal network analog. The procedure is applied to XLPE-insulated high voltage ac cables, subjected to two typical stepwise-constant daily load cycles differing as to the load severity. The application shows that cable life is very sensitive to load cycles, thermal transients and electrothermal synergism, aspects that all deserve attention for estimating accurately the life expectancy of high voltage ac cables in service. The three life models employed, though weighing differently the electrothermal synergism as a consequence of their own functional expressions, give a concordant indication about the possible life extension of cables already in service. Index Terms - Cross linked polyethylene insulation, Electrothermal effects, Failure analysis, Life estimation, Power cables, Reliability estimation. 1 INTRODUCTION IN the free electrical energy market, a target of distribution utilities is to prolong the service period of components beyond design life, if they are working satisfactorily; this holds especially in Europe and North America, where the population of power system components is growing older and older. Such target is being pursued particularly for hardly-accessible devices as high voltage ac (HVAC) cables, whose maintenance and replacement is neither cheap, nor quick and easy. The rated current of HVAC cables involves operation at the rated temperature, i.e. at the “ maximum allowed continuous temperature of cable insulation ” [1-3], and cables are designed under the conservative constraint that their maximum temperature is steady and equal to rated temperature. In fact, load current varies with time following in general daily load cycles and remaining well below rated current for most of cable useful life. Hence, cable maximum temperature is mostly below rated temperature and HVAC cables may live longer than design life at a chosen failure
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This note was uploaded on 06/08/2011 for the course ELECTRICAL 124 taught by Professor Ghjk during the Spring '11 term at Institute of Technology.

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A9F8470Dd01 - 1168 G. Mazzanti: The Combination of...

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